Sorry for the delayed reply, I was away.

It seems to be a quirk in Outlook. If the version is not set to 2.0 (in
my case it was set to less) even though the Product ID was not
Microsoft's (I've written my own engine) the option fails.

It is quite easy to reproduce, export a calendar entry to a file, edit
it using a text editor and reimport it.

Regards,
Al